1. FIELD OF THE INVENTION
This invention relates to a novel relief printing plate, and a method for making the same. More specifically, this invention relates to a relief printing plate in which staining of non-image areas during printing (usually called "bottoming") is prevented as a result of providing small projections in these areas, and to a method for making the same.
2. DESCRIPTION OF THE PRIOR ART
Various methods are in use for making relief printing plates, such as the cast molding of type metal, the etching of a metal plate, or the use of a photopolymer. In all such methods, reliefs which form image areas are required to have considerable height. If reliefs are low in height, in relief printing ink cannot be prevented from adhering to non-image areas and causes bottoming. In order to prevent bottoming, the reliefs are usually required to have a height of at least 0.5 mm.
When printing is carried out using a plate having such a relatively high relief, especially on a rotary press, the printing plate must be mounted in cylindrical form on a plate cylinder. Hence, plates having such high reliefs (that is, thick plates) are inconvenient. For ease of mounting on a rotary press, flexible printing plates are also used. However, in order to mount a printing plate on a plate cylinder, it is preferred that the plate have as small a thickness as possible. Thinner plates also have the advantage of lower costs of production. Furthermore, etching of a metal plate to provide low reliefs can be completed within short periods of time without problems such as side etching, and the etching process can be simplified. In the preparation of a relief printing plate using a photopolymer, on the other hand, that part of the photopolymer layer which corresponds to the relief and which has a thickness corresponding to the height of the relief must be photocured, as described in U.S. Pat. No. 2,760,863 (British Pat. No. 741,441, etc.). Accordingly, when the height of the relief is low, the thickness of the photopolymer layer is small, and therefore, exposure time can be shortened.
Methods for printing with a plate having a relief height as low as about 0.1 to 0.3 mm, known as "dry offset" printing, have recently been regarded as promising. However, a printing press for printing with a printing plate having a low relief height must have a fairly high degree of precision to prevent ink staining of non-image areas, and care must be exercised in the maintenance of the same.
On the other hand, image areas of a lithographic printing plate used for conventional lithographic printing require no relief height. Even if there is some relief height, it is at most several microns, or the image areas are lower in height than non-image areas. In lithographic printing plates, the non-image areas are rendered hydrophilic and water is retained in the non-image areas during printing to prevent the ink staining of these areas. This naturally leads to problems during printing, and various restrictions are imposed on this type of printing. With a view to overcoming such difficulties, lithographic plates which do not use water have been suggested in which non-image areas are made of an ink-repelling substance, such as a silicone resin, and such have come into use to some extent. However, this technique still has disadvantages such as the need for using special ink.
In view of the state of the art, we performed extensive investigations into relief printing plates having relatively low reliefs with which printing can be performed on a conventional relief printing press without staining of non-image areas, which led to the present invention.